Distortion-free amplifier circuit



July 11, 1939.

H. TISCHNER ET AL.

DISTORTION-FREE AMPLIFIER CIRCUIT' Filed Sept. 26, 1936 fig. 2

INVENTbRS Patented July 11, 1939 UNITED STATES PATENT OFFICE Horst Tischner, Berlin-Steglitz,

Joseph Flurl,

Berlin-Karlshorst, and Hermann Wessels, Berlin-Obersehoneweide, Germany, assignors to Allgemeine Elektricitat Gesellschaft,

Germany Berlin,

Application September 26, 1936, Serial No. 102,710 In Germany September 30, 1935 1 Claim.

upper harmonics can be attained in a wide degree in that care is taken by suitable choice of the characteristics and working point and proper dimensioning of the coupling members, that the distortions produced in the first tube are just compensated by those produced in the second tube. This is due to the fact that by the first tube a phase rotation of is produced. Thus if the same upper harmonics are produced in both tubes they compensate each other. Already in this way an extensive freedom from upper harmonics is achieved.

The invention is based upon the discovery that hitherto a complete freedom from upper harmonies could not be attained in cascade amplifiers loaded by complex resistances, in View of the fact that the apparent output resistance of the first tube, substantially determined by the coupling members, is practically a purely real resistance, while the apparent output resistance of the end tube is established by the complex resistance and by the coupling members, so that the said output resistance is, as such, a complex resistance. As is known the operating performance at the anode side of a tube loaded by a complex resistance takes the course of an ellipse. The non-linear distortions therefore depend upon the apparent output resistance, and in the first tube which is substantially loaded by a real resistance these non-linear distortions are different from those of the end tube which is substantially loaded by complex resistances. But a prerequisite for a complete compensation is an identical course in the production of upper harmonies.

To obtain a complete compensation the invention provides a circuit in which in the range to be amplified the frequency course of the apparent output resistance of each tube is as nearly the same as possible as regards value and phase. This can be accomplished in various ways. On the one hand, the coupling between the first tube and the second tube can be chosen in such manner that the course of the apparent resistance of the coupling is equal to that of the complex resistance having series-connected coupling elements ahead thereof, such as for instance a cable with transformer in series thereto. On the other hand, the course of the apparent resistance of the cable having a transformer in series, can be so formed by means of additional circuit elements, that it becomes practically a real resistance within the range to be considered, in the same manner as the ordinary capacity- As is known, in cascade amplifiers, absence of resistance coupling. Finally a compromise can be had between these two possibilities in that on the one hand, the apparent output resistance of the end tube is rendered real as much as possible, and that at the same time, an imaginary component which may still be present, is reproduced in the coupling members between the preliminary tube and the end tube. The frequency course resulting from the entire circuit is eventually freed of distortions by measures taken at the grid of the input tube and which therefore are independent of the compensation.

In the drawing there is shown an example according to the first possibility and an example for the second possibility, whereby Fig. 1 represents a two-stage amplifier cascade, in which the preliminary tube is loaded by an outer resistance, having exactly the same frequency course, and which resistance as regards matching with the preliminary tube is chosen exactly as the output load of the end stage consisting of an output transformer to which a cable is connected. Fig. 2, however, shows an amplifier circuit in which the apparent output resistance of the end tube is rendered real to a wide degree by the addition of circuit elements, such as a condenser.

Fig. 1 shows a two-stage amplifier. The first tube E1 is coupled to the second tube E2 across a complex resistance, while the second tube is loaded by a transformer U, at whose secondary terminals there is placed the substitution circuit of the cable, consisting of a capacitive resistance C1 and ohmic resistance R1 in series to each other. The output transformer in its substitution circuit consists of a relatively large parallel inductance L9 and two smaller stray inductances L81 and LS2. This substitution circuit inclusive its capacitance C2 and ohmic resistance R1 substituting the cable, serves as coupling resistance between the two tubes. The resistances of the substitution circuit are to be modified eventually according to the ratio between the inner resistances of the tube. Since the grid potential is to be applied to the second tube, a second ohmic resistor R2 is to be placed in the circuit, and to avoid placing the plate potential at the grid of the second tube, a second condenser 02 is to be inserted in the circuit.

This condenser and the grid resistance must be r tion circuit as such affords the separation of the plate potentials, and the application of the grid potentials.

In Fig. 2 there is likewise shown a two-stage amplifier circuit. In this circuit, the coupling between the two tubes E1 and E2 is an ordinary capacity-resistance coupling. In order to still obtain practically equal conditions in the apparent resistances in the output of the first tube and in the output of the second tube, there is placed in parallel to the secondary of the transformer U, a correspondingly dimensioned condenser Cz. Fig. 3 shows the eifect of the condenser C3 upon the course of the apparent resistance of the apparent output resistance of the end tube, measured at the terminals of the primary of the transformer U. The dash curve represents the apparent resistance at different frequencies starting from 1.5 kc. (first point of measurement) up to 5.7 kc. (last point of measurement) whereby the condenser C3 is omitted. It is seen that the apparent resistance at low frequencies is practically a real resistance, while however with an increase in frequency it has an imaginary component which steadily increases. The fully drawn curve shows the same mode of measurement whereby however the condenser C3 is included. The curve has greatly shrunk, the apparent resistance is practically a real one, and independent of frequency.

What we claim is:

A system for the amplification of signals with reduced distortion comprising a pair of vacuum tubes, a load circuit included in the output circuit of each tube, means for coupling the output of one tube to the input of the other, the load circuit in one of said tubes being complex in nature and a function of frequency, and the load circuit of the other tube being a resistancereactance network arranged to simulate the variation with frequency, both in magnitude and phase, of the impedance of said preceding named load circuit.

HERMANN WESSELS. JOSEPH FLURL. HORST TISCHNER. 

